The synthesis of new donor-acceptor polymers containing the 2,3-di(2-furyl) quinoxaline moiety: Fast-switching, low-band-gap, p- and n-dopable, neutral green-colored materials

Article abstract of DOI:10.1016/j.electacta.2015.02.033

Three donor-acceptor type π-conjugated polymers were synthesized electrochemically:poly[2,3-di(2-furyl)-5,8-bis(2-(3,4-ethylenedioxythiophene)) quinoxaline] (PFETQ), poly[2,3-di(2-furyl)-5,8-bis(2-thienyl) quinoxaline] (PFTQ) and poly[2,3-di(2-furyl)-5,8-bis(2-(3-methoxythiophene)) quinoxaline] (PFMTQ). All of the synthesized polymers, contained the 2,3-di(2-furyl) quinoxaline moiety in the backbone as the acceptor unit and different thiophene derivatives as the donor units. The electroactivity of the monomers and the electrochemical properties of their polymers were investigated by cyclic voltammetry. The presence of the strong electron-donating ethylenedioxy and methoxy groups on the aromatic structure increased the electron density. Thus, the oxidation potential of FETQ and FMTQ shifted to a lower value than that of FTQ. The optical properties of the polymers were investigated by UV-vis-NIR spectroscopy. Both PFETQ and PFMTQ reveal two distinct absorption bands in the red and blue regions of the visible spectrum, while PFTQ has only one dominant wavelength at 596 nm in the visible region. The colorimetry analysis revealed that while PFTQ has a light blue color, PFETQ and PFMTQ are green in the neutral state. The optical band gaps, defined as the onset of the π-π? transition, were found to be 1.15 eV for PFETQ, 1.2 eV for PFMTQ and 1.34 eV for PFTQ. Moreover, all three polymers showed both n-doping and fast switching times.

Full text of DOI:10.1016/j.electacta.2015.02.033

Electrochimica Acta 160 (2015) 271–280
Contents lists available at ScienceDirect
Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
The synthesis of new donor–acceptor polymers containing the 2,3-di
(2-furyl) quinoxaline moiety: Fast-switching, low-band-gap, p- and
n-dopable, neutral green-colored materials
b,
Zhen Xu a,b, Min Wang c, Weiyu Fan a, Jinsheng Zhao b, , Huaisheng Wang
*
*
a
State Key Laboratory of Heavy Oil Processing, China University of Petroleum (East China), QingDao 266555, PR China
Shandong Key Laboratory of Chemical Energy Storage and Novel Cell Technology, Liaocheng University, Liaocheng, 252059, PR China
b
c
The Central Laboratory of Liaocheng Hospital, 252000 Liaocheng, PR China
A R T I C L E I N F O
A B S T R A C T
Article history:
Three donor–acceptor type
p
-conjugated polymers were synthesized electrochemically:poly[2,3-di(2-
Received 20 September 2014
Received in revised form 4 February 2015
Accepted 4 February 2015
furyl)-5,8-bis(2-(3,4-ethylenedioxythiophene)) quinoxaline] (PFETQ), poly[2,3-di(2-furyl)-5,8-bis(2-
thienyl) quinoxaline] (PFTQ) and poly[2,3-di(2-furyl)-5,8-bis(2-(3-methoxythiophene)) quinoxaline]
(PFMTQ). All of the synthesized polymers, contained the 2,3-di(2-furyl) quinoxaline moiety in the
backbone as the acceptor unit and different thiophene derivatives as the donor units. The electroactivity
of the monomers and the electrochemical properties of their polymers were investigated by cyclic
voltammetry. The presence of the strong electron-donating ethylenedioxy and methoxy groups on the
aromatic structure increased the electron density. Thus, the oxidation potential of FETQ and FMTQ shifted
to a lower value than that of FTQ. The optical properties of the polymers were investigated by UV–vis–NIR
spectroscopy. Both PFETQ and PFMTQ reveal two distinct absorption bands in the red and blue regions of
the visible spectrum, while PFTQ has only one dominant wavelength at 596 nm in the visible region. The
colorimetry analysis revealed that while PFTQ has a light blue color, PFETQ and PFMTQ are green in the
Available online 7 February 2015
Keywords:
2,3-di(2-furyl) quinoxaline
Donor–acceptor type
Optoelectronic properties
Low-band gap polymers
neutral state. The optical band gaps, defined as the onset of the
p–p* transition, were found to be 1.15 eV
for PFETQ, 1.2 eV for PFMTQ and 1.34 eV for PFTQ. Moreover, all three polymers showed both n-doping
and fast switching times.
ã 2015 Elsevier Ltd. All rights reserved.
1. Introduction
polymers are of high interest due to their electrochemical and
optical properties. At present, there have been some published
Since the discovery of the fact that polyacetylene can reach high
electrical conductivity when doped, [1] conductive polymers have
become a major research interest in both the academic and
industry fields. Among the conductive polymers, electrochromic
(EC) polymers, which can reversibly change color by altering their
redox states, have attracted considerable attention over the past
decades. These electrochromic conjugated polymers have been
widely investigated for a variety of applications, such as electro-
chromic devices, [2,3] optical displays, [4] smart windows, [5] and
sensors [6].
reports on the design and characteristics of low-band-gap
conjugated polymers. The most common approach toward the
construction of low band gap systems was employing alternating
strong electron-donating and electron-accepting units in one
polymer, as mixing monomer segments with higher HOMOs and
lower LUMOs is effective in reducing the band gap due to
interchain charge transfer [7,8]. The use of fused aromatics with
electron-withdrawing imine nitrogens (C = N) as acceptor units
and different thiophene derivatives as donor units has drawn great
interest in the design of these alternating systems. Recently, the
use of quinoxaline derivatives as this type of organic electron-
accepting group within donor–acceptor conjugated polymers has
made a great contribution to the electrochromic research field.
It should be noted that polymers of this system usually not only
have low band gap but also can reveal an n-doped character. It is
well-known that only a fraction of the conjugated polymers can
exhibit this n-doped property due to their poor stability at the
reduction potential that leads to the n-doped state. Thus, n-doped
Because the band gap (Eg) is one of the most important factors
controlling the opto-electronic properties of electrochromic
polymers, band-structure engineering has become important in
the field of materials science. Attempts to design low-band-gap
* Corresponding authors.
E-mail addresses: j.s.zhao@163.com (J. Zhao), hswang@lcu.edu.cn (H. Wang).
http://dx.doi.org/10.1016/j.electacta.2015.02.033
0013-4686/ã 2015 Elsevier Ltd. All rights reserved.

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Z. Xu et al. / Electrochimica Acta 160 (2015) 271–280
semiconducting polymers are expected to have a great contribu-
tion to organic electronics in the near future because it will be
possible to manufacture light emitting diodes, bipolar transistors,
and the polymeric analog of silicon field-effect transistors [9].
Thus, conducting polymers with stable negatively doped states
have drawn great attention in the field of electrochromism.
Following this strategy, we previously reported the synthesis of
poly[2,3-di(5-methylfuran-2-yl)-5,8-bis(2-(3,4-ethylenedioxy-
thiophene)) quinoxaline] (PMFEQ), poly[2,3-di(5-methylfuran-2-
yl)-5,8-bis(2-thienyl) quinoxaline] (PMFTQ) and poly[2,3-di(5-
methylfuran-2-yl)-5,8-bis(2-(3-methoxythiophene)) quinoxaline]
(PMFMQ) containing the strong electron-accepting 2,3-di(5-
methylfuran-2-yl) quinoxaline as the acceptor unit. The electro-
chemical band-gaps of these three polymers have been reported to
be between 0.90 and 1.20 eV, and all of them present significant n-
type doping [10].
electrode was an ITO/glass electrode, the counter electrode was a
stainless steel wire, and a Ag wire (0.03 V vs. SCE.) was used as a
pseudo-reference electrode. The polymer films for spectroelec-
trochemistry were prepared by potentiostatic deposition on an
ITO/glass electrode with an active area of 0.9 cm ꢀ 2.0 cm. The
thickness of the polymer films grown potentiostatically on the ITO/
glass was controlled by the total charge passed through the cell.
Colorimetry measurements were obtained by a SP 60 spectropho-
tometer (X-Rite, USA) with illuminator D65 used as the simulated
light source and CIE 10ꢁ as the illuminating/viewing geometry.
Digital photographs of the polymer films were taken by a Canon
Power Shot A3000 IS digital camera.
2.2. Synthesis
2.2.1. 2,3-Bis(2-furyl)-5,8-dibromoquinoxaline (6)
Considering the contribution of the steric interaction between
the repeating units within the polymer backbone, polymers with
more-planar geometries exhibit several special properties and
advantages, including lower band gaps, stronger intermolecular
interactions, and greater degrees of doping [11]. Thus, the 2,3-di(2-
furyl) quinoxaline moiety with a stronger electron-accepting
ability and a greater degree of coplanarity was used as a substitute
for the previous acceptor unit to combine with different thiophene
donors. As a result, three new polymers were electrochemically
synthesized. These were poly[2,3-di(2-furyl)-5,8-bis(2-(3,4-ethyl-
enedioxythiophene)) quinoxaline] (PFETQ), poly[2,3-di(2-furyl)-
5,8-bis(2-thienyl) quinoxaline] (PFTQ) and poly[2,3-di(2-furyl)-
5,8-bis(2-(3-methoxythiophene)) quinoxaline] (PFMTQ). The
amount of decrease in the band gap and the bathochromic shift
of the maximum absorption wavelength of the newly synthesized
polymers containing 2,3-di(2-furyl) quinoxaline demonstrated a
definite improvement in the electrochemical and optical proper-
ties in contrast to the previously reported ones. In addition, it is
worth noting that both PFETQ and PFMTQ showed a green color in
the neutral state and a high transmissivity in the oxidized state.
The effect of the different thiophene derivatives on the photo-
electrical properties was further investigated in detail in this
article.
To a mixture of 3,6-dibromo-1,2-phenylenediamine (1.33 g,
5 mmol) and 1,2-di(2-furyl) ethanedione (0.95 g, 5 mmol) in EtOH
(50 mL), a catalytic amount of p-toluene sulfonic acid (PTSA) was
added, and the solution was then refluxed while being magneti-
cally stirred overnight. At the end of the reaction, a cloudy mixture
was observed. The solution was cooled to 0 ꢁC and filtered. The
separated solid was washed with EtOH several times and dried in a
vacuum oven to give a yellow-green powder (1.8 g, 85.7%). 1H NMR
(CDCl3, 400 MHz, ppm):
d= 7.88 (s, 2H, ArH), 7.63 (d, 2H), 6.97
(d, 2H), 6.60 (q, 2H). (See Supporting Information Fig. S1).
2.2.2. General procedure for the synthesis of FTQ,FETQ, FMTQ
2,3-Bis(2-furyl)-5,8-dibromoquinoxaline (1.68 g, 4 mmol) was
subjected to the Stille coupling reaction with excess tributyl-
stannane (16 mmol) in anhydrous THF (60 mL) using Pd(PPh3)2Cl2
(0.28 g, 0.4 mmol) as the catalyst. After the reaction mixture was
deaerated, the temperature was immediately raised to reflux
temperature. The solution was refluxed with stirring under
nitrogen atmosphere for 24 h, cooled and concentrated on the
rotary evaporator. Lastly the residue was purified by column
chromatography on silica gel using hexane- dichloromethane as
the eluent.
2.2.2.1. 2,3-Di(2-furyl)-5,8-bis(2-thienyl) quinoxaline (FTQ). The
crude mixture was chromatographed on silica gel by eluting
with hexane: dichloromethane (15:1, v/v) to give FTQ as an orange
2. Experimental
2.1. General
solid (1.3 g, 76.5%). 1H NMR(CDCl3, 400 MHz, ppm):
d
= 8.10 (s, 2H,
ArH), 7.90 (d, 2H), 7.62 (d, 2H), 7.54 (d, 2H), 7.20 (q, 2H), 7.12 (d, 2H),
6.63 (q, 2H). 13C NMR (CDCl3, 101 MHz, ppm):
= 148.35, 144.32,
All chemicals were purchased from commercial sources and
used without further purification except for tetrahydrofuran which
was dried and distilled over benzophenone and sodium under
nitrogen before use. The compounds 3,6-dibromo-1,2-phenyl-
enediamine (2), [12] 2-hydroxy-1,2-di(2-furyl) ethanone (4), [13]
1,2-di(2-furyl) ethanedione (5) [14] and tributylstannane com-
pounds [15] were prepared according to the methods described in
the literature. The 1H NMR and 13C NMR spectra of the monomers
were recorded on a Varian AMX 400 spectrometer in CDCl3 at
d
134.85, 133.08, 131.45, 125.74, 123.58, 121.61, 121.59, 119.35, 110.31,
104.43. (See Supporting Information Fig. S2) HRMS (m/z, EI+) calcd
for C24H14N2O2S2 426.5066, found 426.5058.
2.2.2.2.
2,3-Di(2-furyl)-5,8-bis(2-(3,4-ethylenedioxythiophene))
quinoxaline (FETQ). The crude mixture was chromatographed
on silica gel by eluting with hexane: dichloromethane (3:1, v/v) to
give FETQ as a deep red solid (1.6 g, 73.8%). 1H NMR (CDCl3,
400 MHz, and chemical shifts (
d
) were reported relative to
400 MHz, ppm):
(q, 2H), 6.57 (s, 2H), 4.33 (dd, 8H). 13C NMR (CDCl3, 101 MHz, ppm):
= 151.62, 143.95, 141.33, 140.42, 139.35, 136.54, 128.48, 128.16,
113.67, 113.17, 111.90, 103.28, 64.91,64.28. (See Supporting
Information Fig. S3) HRMS (m/z, EI+) calcd for C28H18N2O6S2
542.5782, found 542.5810.
d= 8.58 (s, 2H, ArH), 7.58 (d, 2H), 7.12 (d, 2H), 6.60
tetramethylsilane as the internal standard. Electrochemical
behaviors were investigated by cyclic voltammetry (CV). Electro-
chemical synthesis and experiments were performed in a one-
compartment cell with a CHI 760 C Electrochemical Analyzer under
the control of a computer, employing a platinum wire with a
diameter of 0.5 mm as a working electrode, a platinum ring as a
counter electrode, and a Ag wire (0.03 V vs. SCE.) as a pseudo-
reference electrode. Scanning electron microscopy (SEM) meas-
urements were taken by using a Hitachi SU-70 thermionic field
emission SEM. UV–Vis–NIR spectra were recorded on a Varian Cary
5000 spectrophotometer connected to a computer. A three-
electrode cell assembly was included in which the working
d
2.2.2.3. 2,3-Di(2-furyl)-5,8-bis(2-(3-methoxythiophene)) quinoxaline
(FMTQ). The crude mixture was chromatographed on silica gel by
eluting with hexane: dichloromethane (8:1, v/v) to give FMTQ as a
bright red solid (1.5 g, 77.2%). 1H NMR (CDCl3, 400 MHz, ppm):
d
= 8.02 (s, 2H, ArH), 7.62 (d, 2H), 7.59 (d, 2H), 7.10 (d, 2H), 6.63
(d, 2H), 6.48 (q, 2H). 3.88 (s, 6H). 13C NMR (CDCl3, 101 MHz, ppm):

Z. Xu et al. / Electrochimica Acta 160 (2015) 271–280
273
d
= 153.85, 150.62, 143.80, 139.86, 131.53, 129.82, 125.08, 120.24,
3.2. Electrochemistry
114.51, 107.49, 105.19, 102.34, 58.17. (See Supporting Information
Fig. S4) HRMS (m/z, EI+) calcd for C26H18N2O4S2 486.5582, found
486.5563.
Cyclic voltammetry (CV) is a very useful method that reveals the
electroactivity of monomers and the electrochemical properties of
their polymers. Electrochemical polymerizations of FTQ, FETQ and
FMTQ were carried out by cyclic voltammetry (CV) in an
acetonitrile (ACN)/dichloromethane (DCM) (1:1, by volume)
solvent mixture containing 0.1 M tetrabutylammonium hexafluor-
ophosphate (TBAPF6) as the supporting electrolyte and 0.005 M in
the respective monomers. Voltammetry curves for the repeated
scanning electropolymerization of FETQ are shown in Fig.1 and the
others are given in the Supporting Information. (See Supporting
Information Fig. S5 and Fig. S6)
The first cycle of the CV test corresponds to the irreversible
monomer oxidation at the platinum wire electrode. For the FTQ
monomer, the onset of oxidation is observed at 1.03 V vs. the Ag
wire pseudo-reference electrode. The oxidation potential of FETQ
and FMTQ shifted to 0.79 V and 0.85 V owing to the presence of the
strong electron-donating group on the donor moiety. Comparing
the onsets of monomer oxidation, FETQ and FMTQ were much
more easily oxidized than FTQ. As the cyclic scan continued, the
current densities of the oxidation/reduction peaks increased,
indicating the formation of the conducting polymers on the surface
of the working electrodes [16].
3. Results and discussion
3.1. Synthesis of monomers
The synthetic route to the monomers is presented in Scheme 1.
In the first step, the reduction of 4,7-dibromo-2,1,3-benzothiadia-
zole (1) by NaBH4 as described in the literature [12] yielded the
desired 3,6-dibromo-1,2-phenylenediamine (2). 2-Hydroxy-1,2-di
(2-furyl) ethanone (4) [13] was synthesized by the benzoin
condensation of furfural (3). To a solution of CuSO4 5H2O and
ꢂ
pyridine, 2-hydroxy-1,2-di(2-furyl) ethanone (4) was added, and
then oxidation of 4 gave yellow acicular crystals of 1,2-di(2-furyl)
ethanedione (5) [14]. Then, a simple condensation reaction was
performed with the compounds 2 and 5 to afford the correspond-
ing 2,3-bis(2-furyl)-5,8-dibromoquinoxaline (6). To increase the
reaction yields, p-toluene sulfonic acid was used as the catalyst.
Finally, Stille coupling in the presence of Pd(PPh3)2Cl2 as a catalyst
was used to attach different thiophene donor moieties to the
acceptor quinoxaline unit to give the target monomers FTQ, FETQ
and FMTQ in satisfactory yields (70–80%).
Scheme 1. Synthetic route of the monomers. (a) NaBH4, EtOH, 0 ꢁC, 24 h (b) KCN (c) CuSO4
5H2O, pyridine, reflux, 2.5 h (d) PTSA, EtOH, reflux, overnight (e–g) Pd(PPh3)2Cl2,
ꢂ
tributyl(thiophen-2-yl) stannane (e), tributyl(2-(3,4-ethylenedioxythiophene))stannane (f), tributyl(2-(3-methoxythiophene))stannane (g), dry THF, reflux, 24 h.

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Z. Xu et al. / Electrochimica Acta 160 (2015) 271–280
To study on the electrochemical properties, three polymer films
were prepared on Pt wire by sweeping the potentials for three
cycles. The electrochemical behaviors of the deposited polymers
were investigated at different scan rates between 100 and
300 mV sꢃ1 in monomer-free electrolyte solution. As shown in
Fig. 2, the maximum current density of the three polymers
increased with the scan rate.
The fine electrochemical properties of the D-A type conjugated
polymers were that the polymers can be p-doped/dedoped at
proper positive potentials and n-doped/dedoped at certain
negative potentials. During the p-doping process, the conjugated
chain loses electrons and doping anions could move into the
polymer film.
CP ꢃ eꢃ + Aꢃ $CP+ (Aꢃ)
where CP is the conjugated polymer and Aꢃ denotes the doping
anions.
Fig. 1. Repeated potential scan electropolymerization of FETQ at 100 mV sꢃ1 in
0.1 M TBAPF6/ACN/DCM on a platinum wire electrode (with a diameter of 0.5 mm).
During the n-doping process, the conjugated chain gains
electrons and doping cations could move into the polymer film.
(PFETQ, PFMTQ and PFTQ) were synthesized, and all of them
exhibited distinct but different redox peaks in the n-doping
process. The 2,3-di(2-furyl) quinoxaline moiety used as the
electron-accepting unit is critical to driving this process. In
addition, it is worth noting that the redox peak of the n-doping/
dedoping is much stronger than that of the p-doping/dedoping
process. The results indicate that these three conjugated polymers
are strong electron-acceptors and good n-type conjugated
polymers [18]. Through analysis, the reversible sharp peaks
corresponding to reversible n-doping of the polymers appear in
the range of ꢃ1.5 V to ꢃ1.2 V and the corresponding n-dedoping
peaks appear at approximately ꢃ1.2 V vs. Ag wire.
CP + eꢃ + M+ $ CPꢃ(M+)
where CP is the conjugated polymer and M+ denotes the doping
cations.
As is well known, only a fraction of the conjugated polymers can
exhibit n-doped properties due to their poor stability at the
reduction poten tial. However, a single conjugated polymer system
that contains donor–acceptor units can undergo a transport
mechanism that reduces the degradation effects associated with
water and air, [17] thus enhancing the stabilization of the n-doped
state. For this reason, three donor–acceptor conjugated polymers
Fig. 2. (a) CV curves of the PFETQ film on a platinum electrode at different scan rates between 100 and 300 mV sꢃ1 in the monomer-free 0.1 M TBAPF6/ACN/DCM solution. (b)
CV curves of the PFMTQ film on a platinum electrode at different scan rates between 100 and 300 mV sꢃ1 in the monomer-free 0.1 M TBAPF6/ACN/DCM solution. (c) CV curves
of the PFTQ film on a platinum electrode at different scan rates between 100 and 300 mV sꢃ1 in the monomer-free 0.1 M TBAPF6/ACN/DCM solution.

Z. Xu et al. / Electrochimica Acta 160 (2015) 271–280
275
It is well known that the stability and robustness of the polymer
3.3. Optical properties of the monomers and films
films for long-term multiple redox switching have important
practical applications. Hence, cyclic voltammograms of the
polymers deposited on the Pt electrode were investigated by
potential scanning between neutral and oxidized states in
monomer-free 0.1 M TBAPF6/ACN/DCM at a potential scan rate
of 200 mV sꢃ1. The PFETQ film was tested by cyclic voltammetry at
the applied potential between ꢃ0.8 and 1.0 V with 200 mV sꢃ1 to
evaluate the stability of the device (Fig. 3a). During the above
observation of the ability to switch between oxidized and reduced
states of PFMTQ, 82.1% of its electroactivity is retained after
1000 cycles for the PFMTQ film. PFETQ and PFTQ displayed even
better stability, with 93.8% and 92.6% of the exchange charge still
remaining after sweeping 1000 cycles, respectively. These results
indicate that all three polymers have fine redox stability, and their
long lifetimes mean they have good effective charge compensation
ability.
Likewise, the stability and robustness of the polymer films for
long-term multiple switching between the reduced and neutral
states were also investigated. As seen from Fig. S7, the stability
during the n-doping process was poor. The films were almost
completely dissolved in electrolyte solution after 1000 cycles. Note
that the stability test was carried out in air in order to test the
environmental stability of the polymers because the observed loss
is most probably due to the degradation effects associated with
water and air. Further research is needed under nitrogen
conditions, and the samples should be placed in a completely
dry environment, such as a glove box containing a desiccant.
The UV–vis absorption spectra of FETQ, FMTQ and FTQ
monomers and the corresponding polymer films are shown in
Fig. 4. For these measurements, the polymer films were
potentiostatically electrodeposited onto ITO/glass with the same
polymerization charge of 1.11 ꢀ102C/m2 at 1.0 V, 0.95 V and 1.2 V,
respectively. After polymerization, the films were electrochemi-
cally dedoped at ꢃ0.7 V, -0.6 V and ꢃ0.2 V for 30 s in ACN/DCM (1:1,
by volume) containing 0.1 M TBAPF6, and then washed three times
with ACN/DCM(1:1, by volume) to remove the supporting
electrolyte and oligomers/monomers.
As shown in Fig. 4, the absorption maximum (lmax) of the
monomers FETQ, FMTQ and FTQ due to the
p–p* transition is
centered at 323, 315 and 314 nm, respectively. It can easily be seen
that when thiophene and methoxythiophene units were replaced
by EDOT, the monomer absorbance was shifted to a lower energy
level. The bathochromic shift of the maximum absorption
wavelength of FETQ compared with those of FTQ and FMTQ is
attributed to the strong electron-donating abilities of the ethyl-
enedioxy group that enhances the conjugation effect and decreases
the intensity of the
p–p* electronic transition absorption. This
trend was also observed in the absorbance values of the polymer
films. In their neutral state, both PFETQ and PFMTQ showed
maximum absorption at approximately 790 nm in the visible
region due to the
p–p* transition, while the maximum absorption
wavelength of PFTQ was located at 595 nm. Additionally, because
the conjugation length was increased, all of the dominant
Fig. 3. (a) Electrochemical stability of PFETQ during p-doping process in the monomer-free ACN/DCM containing 0.1 M TBAPF6 solutions after 1000 switching by the CV
method. (Pt working electrode versus Ag wire pseudo reference electrode.) (b) Electrochemical stability of PFMTQ during p-doping process in the monomer-free ACN/DCM
containing 0.1 M TBAPF6 solutions after 1000 switching by the CV method. (Pt working electrode versus Ag wire pseudo reference electrode.) (c) Electrochemical stability of
PFTQ during p-doping process in the monomer-free ACN/DCM containing 0.1 M TBAPF6 solutions after 1000 switching by the CV method. (Pt working electrode versus Ag wire
pseudo reference electrode.)

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Z. Xu et al. / Electrochimica Acta 160 (2015) 271–280
furyl) quinoxaline moiety when this was used as a substitution.
The introduction of the 2,3-di(2-furyl) quinoxaline acceptor
increases the double bond character which leads to a broadening
of the valence and conduction bands and induces small band gaps
[24–27]. Table 1 clearly summarizes the maximum absorption
wavelength (lmax), the absorption onset wavelength (lonset), the
optical band gap (Eg), and the HOMO/LUMO energy levels of the
monomers and corresponding polymers.
3.4. Morphology
The morphologies of the polymer films were closely related to
the electrochemical properties of the polymers. Thus in the present
paper, scanning electron microscopy (SEM) was employed as a
useful method to investigate the surface morphologies of the
polymer films. Available films were prepared potentiostatically on
ITO/glass with the same polymerization charge of 5.55 ꢀ102C/m2
in the ACN/DCM (1:1, by volume) solution containing 0.1 M TBAPF6
and 0.005 M in the relevant monomer. In addition, the thickness of
the PFETQ, PFMTQ and PFTQ films were investigated by step
profilers, which were 776 nm, 710 nm and 824 nm, respectively. All
of the polymers were dedoped before characterization. Fig. 5
clearly displays the SEM images of these polymer films. An
accumulation state of small globules with interlinked holes among
the clusters is observed on the surface of the PFETQ polymer film
(Fig. 5a). The PFTQ film (Fig. 5b) exhibits similar surface
morphology to PFETQ. As for PFMTQ film, there are pervasive
gibbosities on the surface and some gullies are also found (Fig. 5c).
The morphologies of the three polymers were quite different from
each other, which might be an indication of the different
aggregation structure of the conjugated polymers.
Fig. 4. (a) UV–vis absorption spectra of FETQ, FMTQ and FTQ dissolved in
dichloromethane. (b) absorption spectra of the corresponding polymers on an ITO-
coated glass slide.
3.5. Spectroelectrochemical properties of the polymer films
wavelengths were shifted to the longer wavelength region for
polymers PFETQ, PFMTQ and PFTQ compared to their respective
monomers, which verifies that the longer the wavelength of
maximum absorption is, the higher the conjugation length in the
polymer will be [19].
Spectroelectrochemistry is a useful method for studying the
changes in the absorption spectra and information about the
electronic structures of conjugated polymers as a function of the
applied potential difference [28]. In situ electronic absorption
spectra studies of the polymer films were performed using
stepwise oxidation in a monomer-free 0.1 M TBAPF6/ACN/DCM
solution. Spectroelectrochemistry and the corresponding colors
of the polymeric films in the n-dopable, neutral and p-dopable
states are shown in Fig. 6. The PFETQ, PFMTQ and PFTQ films
were electrodeposited onto ITO/glass electrode with the same
polymerization charge of 1.11 ꢀ102C/m2 under constant poten-
tials of 0.8 V, 1.1 V and 1.2 V, respectively. After polymerization,
electrochemical dedoping was carried out at ꢃ0.8 V, ꢃ0.7 V and
ꢃ0.2 V, respectively, for 30 s in 0.1 M TBAPF6/ACN/DCM solution,
and the samples were then washed three times using ACN/DCM
(1:1, by volume) to remove the supporting electrolyte and
oligomers/monomers.
A
regular alternation of conjugated donor and acceptor
moieties along the polymer backbone increases the double-bond
character and establishes an intramolecular charge transfer,
allowing absorption of low-energy photons [20–22]. Low-band-
gap polymers have been defined as conjugated polymers with a
band gap below 1.5 eV [23]. The low optical band gap values
(calculated from the onset of
p–p* transition) of PFETQ (1.15 eV),
PFMTQ (1.20 eV) and PFTQ (1.34 eV) indicate that the three newly
synthesized polymers all have low band gaps. By contrast with the
reported polymer films containing 2,3-di(5-methylfuran-2-yl)
quinoxaline as the acceptor,10 the band gaps of PFETQ, PFMTQ
and PFTQ shift to a much lower level due to the stronger electron-
accepting ability and well-defined coplanarity of the 2,3-di(2-
Table 1
The onset oxidation potential (Eonset), maximum absorptionwavelength (lmax), absorption onsets wavelength (lonset), optical band gap (Eg), HOMO/LUMO energy levels of the
monomers and corresponding polymers.
Polymer
Eonset, vs.Ag (V)a
lmax (nm)
lonset (nm)
Eg,op (eV)a
HOMOb (eV)
LUMOc (eV)
PFETQ
ꢃ0.39
ꢃ0.22
0.31
ꢃ0.40
ꢃ0.28
0.21
790
790
596
780
790
558
1078
1033
925
986
1014
753
1.15
1.20
1.34
1.26
1.22
1.65
ꢃ4.04
ꢃ4.21
ꢃ4.74
ꢃ4.03
ꢃ4.15
ꢃ4.64
ꢃ2.89
ꢃ3.01
ꢃ3.40
ꢃ2.77
ꢃ2.93
ꢃ2.99
PFMTQ
PFTQ
PMFEQd
PMFMQd
PMFTQd
a
Calculated from the low energy absorption edges (lonset), Eg = 1240/lonset
HOMO = (Eonset + 4.4) (Eonset vs. SCE).
Calculated by the subtraction of the optical band gap from the HOMO level.
Data were taken from Ref. [10].
.
b
c
d

Z. Xu et al. / Electrochimica Acta 160 (2015) 271–280
277
region due to the
p
–
p
* transition at neutral state. The intensity of
the absorption band decreases and a new inconspicuous absorp-
tion band in the NIR region arises due to the formation of charge
carriers upon oxidation of the PFTQ. Meanwhile, the PFTQ film
turned to a tile blue color at its p-doped state after being fully
oxidized.
To obtain
a green color there should be at least two
simultaneous absorption bands in the red and blue regions of
the visible spectrum, and these bands should also be controlled by
the same applied potential. All this complexity is expected to result
in only a limited number of neutral-state green polymeric
materials in the literature [29,30]. Fortunately, in its neutral state,
PFETQ showed maximum absorption peaks at 458 nm and 790 nm
in the visible region which are essential values to maintain a green
color. Likewise, PFMTQ film with two absorption maxima at
approximately 427 nm and 790 nm also reveals a green neutral
state. Through the contrast analysis of the absorption values of the
polymers, the strong electron-donating ethylenedioxy and
methoxy groups affect not only the monomer oxidation and the
polymer redox couple potentials but also the maximum absorption
wavelengths of the corresponding transitions. Favorable donor–
acceptor matches with the quinoxaline moieties of PFETQ and
PFMTQ increased the double-bond character, creating an intramo-
lecular charge transfer, allowing absorption of low-energy photons
[20–22] and producing a bathochromic shift of the maximum
absorption wavelengths. For both polymers, the
p–p* electronic
transition absorption decreased while charge-carrier absorption
bands evolved at approximately 1000 nm for PFETQ and 1065 nm
for PFMTQ at moderate oxidation potentials. The appearance of
charge carrier bands could be attributed to the evolution of polaron
and bipolaron bands [31]. Finally, no significant absorption peaks
can be found in the visible region for PFETQ and PFMTQ, which
resulted in a highly transmissive colorless oxidized state. However,
the absorption of all three polymers increased substantially in the
near infrared region owing to the charge carriers.
The color changes were further investigated by colorimetric
parameters and color spaces using the CIE 1976 L*a*b* with the
value of L* representing lightness and the opponent color
dimensions (a* and b*) are the red-green balance for a* and
yellow-blue balance for b* [32]. Colorimetry measurements are
obtained using an SP 60 spectrophotometer (X-Rite, USA). The
PFTQ is light blue (L*, 43.2; a*, 14.0; b*, ꢃ45.6) in the neutral state,
while the oxidized state becomes light gray (L*, 58.6; a*, ꢃ6.9; b*,
13.4). The PFETQ film changed from a green neutral state (L*, 63.3;
a*, ꢃ42.4; b*, 21.1) to a transmissive colorless oxidized state (L*,
59.0; a*, ꢃ2.4; b*, 11.7). Although the PFMTQ film also revealed a
green neutral state (L*, 55.3; a*, ꢃ46.2; b*, 17.9) and a highly
transmissive colorless oxidized state (L*, 57.3; a*, -3.6; b*,10.4), the
colorimetric parameters were different.
Although the conjugated polymers have considerable potential
to be n-doped, only a small portion of them can exhibit this
property because of the easy degradation reaction associated with
water and air [17]. The use of the donor–acceptor approach is one
of the significant methods to enhance the stabilization of the n-
doped state and display the n-doped characters of the conjugated
polymers. Additionally, the n-doping of a conjugated polymer
system in an n-type manner is not only an electrochemical
formation of a reduced state. The introduction of charge carries to
conjugated systems should yield considerable structural, conduc-
tivity and optical differences [33]. Hence, to prove that an n-type
doping process is present, both the electrochemistry of the
reduced state and moreover, the spectral changes that occur upon
reduction should be examined [34]. To test this hypothesis, UV–vis
absorption spectra of three polymers at the reduction potential of
ꢃ1.6 V were investigated.
Fig. 5. (a) SEM images of PFETQ deposited potentiostatically onto ITO/glass
electrode. (b) SEM images of PFTQ deposited potentiostatically onto ITO/glass
electrode. (c) SEM images of PFMTQ deposited potentiostatically onto ITO/glass
electrode.
Most of the conjugated polymers are colored in the neutral state
because the energy difference between the bonding and the
antibonding orbitals (the
p–p*transition) occurs in the visible
region. When oxidized, the lower energy transitions become more
intense, and a second color appears. To obtain different UV–vis
absorbance curves of the PFTQ, various potentials ranging from
ꢃ0.2 V to 1.2 V were applied. As seen from Fig. 6, PFTQ exhibits a
blue color with only one absorption band at 596 nm in the visible

278
Z. Xu et al. / Electrochimica Acta 160 (2015) 271–280
Fig. 6. (a) Spectroelectrochemistry of PFETQ films on ITO/glass electrode at different applied potentials in the monomer-free 0.1 M TBAPF6/ACN/DCM solution. (b)
Spectroelectrochemistry of PFMTQ films on ITO/glass electrode at different applied potentials in the monomer-free 0.1 M TBAPF6/ACN/DCM solution. (c)
Spectroelectrochemistry of PFTQ films on ITO/glass electrode at different applied potentials in the monomer-free 0.1 M TBAPF6/ACN/DCM solution.
As seen from Fig. 6, the absorption spectra of the reduced forms
were markedly different from that of the other two states. The
changes in UV–vis absorption spectra were also reflected in a
corresponding change in the color of the three films. For PFETQ,
two coterminous absorption maxima were found at 423 and
538 nm, which resulted in a deep purple-red reduced state (L*,
47.5; a*, 45.2; b*, ꢃ21.6). When a potential of ꢃ1.6 V was applied to
polymer PFMTQ, the film became light purple-red in color (L*, 51.3;
a*, 32.8; b*, ꢃ13.3) with two separated absorption peaks at 443 and
626 nm. As for the PFTQ, the single absorption band at 537 nm in
the visible region resulted in a bronze color (L*, 52.7; a*, 12.1; b*,
25.4), which indicates a significant difference from that of the
neutral and p-doped states. The electrochemical property of the
reduced state and the spectral changes that occur upon reduction
proved that the n-doping process truly occurred.
time, another very important characteristic of electrochromic
materials, is the time necessary for 95% of the full optical switch
(after which the naked eye could not sense the color change) [35].
Fig. 7a shows the switching properties of PFETQ between -0.7 and
0.8 V at two different wavelengths both in the visible and NIR
regions. At 458 nm, the optical contrast for PFETQ was calculated as
30% and the switching time was 0.9 s. The polymer switches
rapidly and achieves 87% of its total optical change in 0.8 s at
1600 nm.
PFMTQ was also switched from ꢃ0.6 V to 1.1 V at 5 s step
intervals, while the change in transmittance at three different
wavelengths was monitored. The optical contrasts for PFMTQ were
calculated as 20% at 442 nm, 26% at 780 nm, and 81% at 1600 nm.
PFMTQ revealed switching times of 0.45 s at 442 nm and 1600 nm,
0.9 s at 780 nm (Fig. 7b). The PFTQ revealed a 21% transmittance
change upon doping/de-doping process at 590 nm and 76% at
1336 nm (Fig. 7c). The switching time was calculated as 0.8 s and
0.4 s at corresponding wavelengths from the reduced to the
oxidized state. In addition, all of the polymers show high optical
contrasts in the NIR region, which is a very significant property for
various NIR applications.
3.6. Switching properties
Polymers with the ability to switch rapidly and exhibit striking
color changes according to the alternate reduced and oxidized states
are crucial for electrochromic applications. A repeated potential
stepping method coupled with optical spectroscopy was used to
observe switching times and optical contrasts for these polymers.
Electrochromic switching of the polymers was performed at regular
intervals of 5 s in a monomer-free ACN/DCM (1:1, by volume)
solutioncontaining0.1 M TBAPF6 as a supportingelectrolyte. Asseen
from Fig. 7, the behaviors included only slight losses in the percent
transmittance contrast value after regular switching during 300 s,
indicating a certain stability of all three films.
The coloration efficiency (another important characteristic of
electrochromic materials) has been used to investigate the optical
behavior comparison of polymer films. It is defined as the change
in the optical density (DOD) for the charge consumed per unit
electrode area (DQ) [36]. The corresponding equations are given
below [37]:
ꢀ
ꢁ
Tb
Tc
DOD
Q
D
OD ¼ 1g
and
h
¼
D
In these studies, the optical contrast (DT%) of the polymer films
where Tb and Tc are the transmittances before and after coloration,
respectively, Q is the amount of injected charge per unit sample
which can be defined as the transmittance difference between the
redox states was recorded at constant wavelengths. The response
D

Z. Xu et al. / Electrochimica Acta 160 (2015) 271–280
279
Fig. 7. (a) Electrochromic switching, percent transmittance change monitored at 458 and 1600 nm for PFETQ on ITO/glass electrode between ꢃ0.7 V and 0.8 V. (b)
Electrochromic switching, percent transmittance change monitored at 442, 780 and 1600 nm for PFMTQ on ITO/glass electrode between -0.6 V and 1.1 V. (c) Electrochromic
switching, percent transmittance change monitored at 590 and 1336 nm for PFTQ on ITO/glass electrode between -0.2 V and 1.2 V.
area and
h
denotes the CE (coloration efficiency). The CE of the
Cꢃ1 at 458 nm. The
spectroelectrochemical properties of the resulting electropoly-
merized materials. Polymers with the stronger EDOT or methox-
ythiophene donor unit have lower oxidation potentials and band
gaps relative to the thienyl derivative. In addition, PFTQ displayed
two different colors (tile blue in the neutral state, light gray in the
oxidized state), while favorable donor–acceptor matches with the
quinoxaline moieties of PFETQ and PFMTQ resulted in a green
neutral state and a highly transmissive colorless oxidized state. As
PFETQ film was calculated to be 157.1 cm2
ꢂ
PFMTQ filmwas calculated to have a CE of 130.4 cm2 Cꢃ1 at 442 nm,
ꢂ
and the PFTQ had a CE of 90.8 cm2
ꢂ
Cꢃ1 at 590 nm. The +/ꢃ error of
all three polymers in the coloration efficiencies was calculated to
be lower than 1.0 cm2 Cꢃ1. The results clearly demonstrate that the
ꢂ
coloration efficiencies of PFETQ and PFMTQ are much greater than
that of PFTQ.
By contrast with the reported polymer films containing 2,3-di
(5-methylfuran-2-yl) quinoxaline as the acceptor [10], PFETQ and
PMFEQ with the same ethylenedioxythiophene donor unit but
different acceptors, showed a similar response time, while the
a donor–acceptor-donor type of
showed high optical contrast (
p
-conjugated polymers, they all
D
T%), fast response time and good
cyclic voltammetry stability. In addition, the generation of redox
waves in CV at negative potentials and variation of the spectral
absorption curves upon reduction proved that all three polymers
have stable n-doping properties. The polymer films with their
favorable electrochemical and optical properties are expected to be
useful for practical use in electrochromic display applications.
PFETQ film displayed a prominently higher optical contrast (DT%)
than PMFEQ. Analogously, the PFMTQ film exhibits fast response
time, despite the fact that the optical contrast is a little inferior to
that of PMFMQ. A comparison between the PFTQ and PMFTQ films
reveals a higher optical contrast and faster response time for PFTQ.
Generally speaking, the switching properties of the newly
synthesized polymers were superior to the previously reported
ones [10] due to their stronger electron-accepting ability and the
well-defined coplanarity of the 2,3-di(2-furyl) quinoxaline moiety
that was used as a substitution. In light of the superior character-
istics mentioned above, PFETQ, PFMTQ and PFTQ could be the
better candidates for electrochromic display applications.
Acknowledgements
The work was financially supported by the National Natural
Science Foundation of China (51473074, 31400044), the General
and Special Program of the postdoctoral science foundation China
(2013M530397, 2014T70861) and the Fundamental Research
Funds for the Central Universities (15CX06049A).
4. Conclusion
Appendix A. Supplementary data
In this study, three monomers based on 2,3-di(2-furyl)
quinoxaline as the acceptor unit were synthesized to understand
the effects of donor strength on the electrochemical and
Supplementary data associated with this article can be found, in
the
online
version,
at
http://dx.doi.org/10.1016/j.
electacta.2015.02.033.

280
Z. Xu et al. / Electrochimica Acta 160 (2015) 271–280
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